1. Technical Field
The invention relates to cable television (CATV) interdiction systems and, more particularly, to a method and apparatus for controlling the jamming parameters in such systems.
2. Description of the Prior Art
In the past, a scrambler has been provided to encode premium television channels at a headend of a cable television system. The applied scrambling precluded reception by an unauthorized converter/decoder at a connected premises. Data representing the channels or tiers of programming to which the subscriber was entitled were addressably transmitted to a particular converter/decoder and stored in an authorization memory. As a result of the addressed transmission, a subsequently transmitted program would be authorized by selectively enabling the decoder portion of the converter/decoder to decode the scrambled premium channel or program.
The provision of one scrambler per premium channel at the headend and the inclusion of a descrambler in each converter/decoder at the premises of the television receiver was particularly expensive. Furthermore, providing a converter/decoder on premises has turned out to be a great temptation to service pirates who imaginatively seek ways to receive premium channels. As a result, cable television equipment manufacturers have entered into a veritable war with such pirates resulting in complicated service authorization protocols, which in some instances involve multiple layers of encryption by both in-band and out-of-band data transmissions thereby further increasing the costs of the converter/decoder. In addition, scrambling systems may leave artifacts in the final signal.
Consequently, the cable industry has reviewed other technology developed in the early stages of cable television, such as the application of negative and positive traps, and more recent techniques, such as interdiction, to improve CATV systems.
A relatively recent technique for premium channel control is the interdiction system, so called because of the introduction of an interfering signal into a premium channel at the subscriber's location. Most embodiments consist of a pole-mounted enclosure located outside the subscriber's premises designed to serve one or more subscribers. This enclosure contains at least one microprocessor controlled oscillator and switch control electronics to secure several television channels. Control is accomplished by injecting an interfering or jamming signal into unauthorized channels from this pole-mounted enclosure.
For the sake of efficiency, it is known to utilize one oscillator to jam several premium television channels. This technique not only reduces the amount of hardware required, but also maximizes the system flexibility. The jamming signal frequency is moved as a function of time from channel to channel. The oscillator is frequency agile and hops from jamming one premium channel frequency to the next. Cable television channels and, of course, premium service channels may extend over a wide range of frequencies, for example, from 54 Mhz. up to and including 1000 Mhz. Thus, if only one oscillator were provided, it would have to be frequency agile over a wide range.
One such system, illustrated in U.S. Pat. No. 4,450,481 by Dickenson, has a single frequency agile oscillator which provides a hopping gain-controlled jamming signal output to four high frequency electronic switches. In this system, each switch is associated with one subscriber drop. Under microprocessor control and depending on which subscribers are authorized to receive transmitted premium programming, the microprocessor selectively gates the jamming signal output of the single oscillator via the switches into the path of the incoming broadband television signal to each subscriber. Consequently, an unauthorized subscriber upon tuning to a premium channel will receive the premium channel on which a jamming signal of approximately the same frequency has been superimposed.
A significantly more advantageous interdiction system is disclosed in U.S. Pat. No. 4,912,760 by West, Jr., et al. which controls a plurality of frequency agile oscillators for each subscriber. Each voltage controlled oscillator is allocated a continuous band of frequencies consistent with the elimination of jamming signal harmonics which could disturb authorized programming at a higher channel frequency. The interdiction apparatus includes generating and storing frequency control words for operating the voltage controlled oscillators consistent with a headend selected jamming factor for a particular channel to be jammed and addressably transmitted and stored premium programming authorization data. In U.S. application Ser. No. 07/476,041, filed Feb. 6, 1990, by West, Jr., et al., the method of programming the time slots of the West I system to vary the dwell time and jamming factor of the jamming signals for different premium channels was disclosed.
The system of West I and West II uses several variable frequency oscillators to cover the spectrum of the CATV broadband signal, in general from 54 Mhz. to 1000 Mhz. or higher. The lower frequencies of this spectrum are harder to cover because a normal oscillator covers less bandwidth at the lower frequencies. This has not been a significant problem in the past because the lower frequencies, usually channels 2-12, are VHF broadcast channels and supplied to the subscribers without restriction. The premium channels which required interdiction were higher in frequency and within the bandwidths of the several VCOs.
However, with the extension of CATV system bandwidth, initially about 330 Mhz, and then expanded to 550 Mhz, 750 Mhz, and now to 1000 Mhz. and above, the bandwidth of a small plurality of oscillators is again being taxed. Moreover, some CATV systems are placing premium channels in the VHF spectrum and some are using tiered service where VHF channels are becoming pay type viewing. To overcome the bandwidth expansion on both ends of the spectrum it would be advantageous for an interdiction system to be able to control more than one range of voltage controlled oscillator. Voltage controlled oscillators which are more efficient in one use than another because of their operation over a particular frequency range can be of different types. In addition to controlling different ranges of VCOs it would be advantageous for an interdiction apparatus to control different types of VCOs to provide system flexibility.